1. Field of the Invention
The present invention relates to a compact zoom eyepiece optical system applied to binoculars, astronomical telescopes, or the like.
2. Description of the Related Art
Generally, a zoom eyepiece optical system, which is used in combination with an objective optical system, includes a negative powered (hereinafter, negative) first lens group which is movable upon zooming, a positive powered (hereinafter, positive) second lens group which is movable upon zooming, and a stationary positive third lens group, in this order from the objective optical system. According to the above arrangement, an intermediate image is formed between the negative first group and the positive second lens group. An optical system, such as ones described above, have been disclosed in, for example, in Japanese Unexamined Patent Publication Nos. Sho-62-134617 and Hei-6-175048.
In the above-described conventional zoom eyepiece optical system, the negative first lens group is provided on the object-side of the intermediate image, so that even at the long focal length extremity, the zoom eyepiece optical system can magnify the image formed by the objective optical system. Accordingly, the combined focal length of the positive second lens group and the positive third lens group needs to be made longer with respect to the focal length of the entire optical system. Consequently, the lens diameter becomes considerably large, compared with, e.g., a single focal-length eyepiece lens system having the same focal length.
The present invention provides a compact zoom eyepiece optical system having a small lens diameter.
According to an aspect of the present invention, there is provided a zoom eyepiece optical system, which is used in combination with an objective optical system, including a first lens group which is stationary with respect to the objective optical system, a negative second lens group which is movable upon zooming, a positive third lens group which is movable upon zooming, and a stationary positive fourth lens group, in this order from the objective optical system. Upon zooming from the long focal length extremity to the short focal length extremity of the zoom eyepiece optical system, the negative second lens group and the positive third lens group move away from each other, and an intermediate image is formed between the negative second lens group and the positive third lens group.
The zoom eyepiece optical system according to the present invention preferably satisfies the following conditions:
0.6 less than ml less than 0.95xe2x80x83xe2x80x83(1)
xe2x88x920.05 less than fL/fl less than 0.5xe2x80x83xe2x80x83(2)
wherein
ml designates the lateral magnification of the first lens group when the diopter of a bundle of light rays emitted from the zoom eyepiece optical system is xe2x88x921 (l/m);
fL designates the focal length of the entire zoom eyepiece optical systems at the long focal length extremity; and
fl designates the focal length of the first lens group.
The zoom eyepiece optical system can satisfy the following condition:
0.6 less than fS3-4/fL less than 1.0xe2x80x83xe2x80x83(3)
wherein
fS3-4 designates the combined focal length of the positive third lens group and the positive fourth lens group at the short focal length extremity.
The zoom eyepiece optical system preferably satisfies the following condition:
0.9 less than fL3-4/fL less than 1.2xe2x80x83xe2x80x83(4)
wherein
fL3-4 designates the combined focal length of the positive third lens group and the positive fourth lens group at the long focal length extremity.
The first lens group preferably includes a single lens element for the purpose of cost reduction.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2002-116551 (filed on Apr. 18, 2002) which is expressly incorporated herein in its entirety.